For persons who cannot benefit from traditional, air conduction hearing aids there are other types of sound transmitting hearing aids on the market, ie bone anchored hearing aids which mechanically transmit the sound information to a persons inner ear via the skull bone by means of a vibrator. The hearing aid device is connected to an anchoring element in the form of an implanted titanium screw installed in the bone behind the external ear and the sound is transmitted via the skull bone to the cochlea (inner ear), ie the hearing aid works irrespective of a disease in the middle ear or not. The bone anchoring principle means that the skin is penetrated which makes the vibratory transmission very efficient.
This type of hearing aid device has been a revolution for the rehabilitation of patients with certain types of impaired hearing. It is very convenient for the patient and almost invisible with normal hair styles. It can easily be connected to the implanted titanium fixture by means of a bayonet coupling or a snap in coupling. One example of this type of hearing aid device is described in U.S. Pat. No. 4,498,461 and it is also referred to the BAHA® bone anchored hearing aid marketed by Entific Medical Systems in Göteborg.
The fixtures which are used today for the bone anchored hearing aid devices are normally designed in such a way that a screw tap is required to form an internal thread in the hole drilled in the skull bone before the screw is inserted. One example of such a fixture is illustrated in U.S. Pat. No. Des. 294,295. This fixture has an external thread with small cutting edges which have a scraping effect in the pre-tapped bone hole. The fixture has also a flange which functions as a stop against the bone surface when the fixture is screwed down into the skull bone. The flange is also in this case provided with through holes for bone ingrowth or the like.
However, it is also previously known to use self-tapping fixtures for the hearing aids. The advantage with that type of fixtures is that they can be inserted without the use of any screw tap, see SE 0002627-8. The installation of the implant is then much easier as at least one tool and one step in the installation procedure of the implant is eliminated.
The implants which are used on the market today are normally in two pieces, one piece consists of the screw-shaped anchoring element (fixture) and the other piece consists of the abutment sleeve for skin penetration. The reason for this two-piece design is the fact that the surgical technique which normally has been used for installing the implants has been carried out as a two-step procedure. In the first step the fixture is inserted and maintained unloaded during a healing period of some months or so. After this healing period the second step of the surgical procedure, ie the connection of the abutment sleeve by means of a screw connection, is carried out.
Thanks to this two-part design the implants can be up-graded if necessary without removing the fixture, and if the abutment sleeve is damaged then it can also be replaced without need of removal of the bone anchored screw.
The disadvantage with these two-part implants is the fact that the number of individual pieces to handle is increased and thereby the surgery time. Normally the fixture is installed by means of a so-called fixture mount which is attached to the fixture by means of a screw joint and the fixture mount has to be removed after the fixture has been inserted. After this moment the abutment sleeve has to be attached in correct position to the fixture by means of a very small screw, either directly after the insertion of the fixture or after a suitable healing period. In both cases there is a risk that the abutment sleeve is attached to the fixture with a too small tightening torque (then there is a risk that the screw joint is loosening) or a too big tightening torque (then there is a risk that the anchorage of the fixture screw in the bone is jeopardized).
By SE 9702164-6 it is previously known to integrate a flange fixture with a first coupling part so that an integral one-piece member is formed. The disadvantage with such an integral implant is the fact that a deformation zone has to be arranged between the flange fixture part and the coupling part of the implant. This deformation zone has at the same time the function of a dismounting zone within which the first coupling part can be separated from the implant by means of a specific tool (cylindric cutter) if a dismounting of the main parts of the implant should be necessary. In order to be able to re-connect the parts with each other it is necessary to provide a washer to bridge the milled away portion of the implant. The simplified installation procedure with such an implant is then counteracted by the complicated procedure which is required if a dismounting should be necessary.